1. Field of the Invention
The invention relates to the art of forming images on a charge retentive surface and more particularly to a method and apparatus for uniformly charging a charge retentive surface and/or discharging the surface.
2. Description of the Prior Art
Pursuant to forming images on a charge retentive surface, it is common to apply a uniform electrostatic charge to the surface of a charge retentive surface, for example, a photoconductive layer. The charge in selected areas is then dissipated by exposing the surface to a light image to form an electrostatic latent image. The latent image is then rendered visible by applying thereto finely divided electrostatically charged developer particles which adhere to the surface by electrostatic attraction. Permanent visible images can be obtained, for example, by using thermoplastic developer particles which are heat fused to a copy substrate such as plain paper.
Charging is conventionally accomplished by exposing the surface of the charge retentive surface to a corona source, the polarity of which is chosen to produce the desired results upon the particular surface being charged. The corona source is commonly provided by one or more fine wires positioned close to the surface. When a high voltage potential is applied to the wire or wires, a corona is generated or discharged and ions are attracted to and deposited on the surface. Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the surface before imaging.
High voltages for generating corona are particularly desirable for producing charge uniformity, but can subject the surface to excessive charge build-up (charge potential), which can cause damage by current leakage. A number of techniques have been employed to limit the charge potential on the photoconductive layer. For example, complex electrical circuitry has been used to limit corona production (an example being disclosed in U.S. Pat. No. 3,335,275 to King).
Another technique employed to limit the charge potential on a surface is the use of a wire grid or screen placed between the corona discharge wire and the surface. This apparatus is commonly referred to as a "scorotron" and is described in U.S. Pat. No. 2,777,957. The grid is maintained at a predetermined potential and serves to terminate further charging of the surface when the surface potential on all portions of the surface corresponds to the grid potential. The grid can be grounded or biased by means of an external voltage source, or it can be self-biased from the corona current by connecting the grid to ground arrangement through current flow restricting devices (an example of the latter being illustrated in U.S. Pat. No. 3,729,649). In U.S. Pat. No. 3,729,649 a control electrode or grid is connected to ground through a zener diode. Such a grid to ground arrangement is also disclosed in U.S. Pat. No. 4,233,511 assigned to Ricoh Company Limited. In such an arrangement, the threshold voltage for conduction in the reverse bias direction determines the voltage value to which the voltage on the control grid is controlled. The potential to which the grid is controlled determines the voltage level to which the charge retentive surface is charged.
Due to inherent manufacturing tolerance variations in devices such as zener diodes, it is not always possible to control the scorotron grid to the exact voltage desired. This problem has been, to a degree, overcome as illustrated in U.S. Pat. No. 4,335,420 by the provision of plural zener diodes and a multi-position switch which serves to connect one or the other of these diodes to the control grid or screen. However, if the desired voltage level for the control grid is not exactly matched by the voltage of one of the zener diodes then this arrangement is not satisfactory. Moreover, this arrangement is not entirely suitable for varying the voltage to which the grid is controlled when this value has to be changed because of a change in voltage level of the surface. As the charge retentive surface ages its charging characteristics change thereby necessitating a change being made to the output of the charging device. Furthermore, charging characteristics of one retentive surface to another vary as a result of manufacturing tolerances necessitating individual adjustment of the charging device for each surface.
Adjusting the self-bias on scorotron grid can be effected by means of a variable resistor interposed between the control grid and ground. However, this arrangement is not suitable for this application where there are current fluctuations due to, for example, line voltage surges. This is because such current variations produce large grid voltage variations. A more serious problem with respect to current variations arises from variations in voltage of the incoming charge receptor which translate into grid current variations in the scrotoron.
In view of the foregoing, it can be seen that a charging device such as a scorotron that is provided with means for compensating for variation in component tolerances as well as photoconductive layer changes while substantially maintaining the grid voltage at a constant level and which does not adversely affect the photoconductive layer is most desirable. This is particularly true in the case of scorotron devices of the type disclosed in U.S. patent application in Ser. No. 567,717, filed Jan. 1, 1984 in the name of Gundlach et al. and assigned to the same assignee as the instant application. In the aforementioned application improved charging of a charge retentive surface is effected by a device that is more closely spaced to the charge retentive surface than prior art devices and wherein the open area in the screen or grid is less than prior art screens. In such a device the grid exerts more powerful control over the final surface potential.